Automatic transfer switches are used to vary the source of electrical power for a particular consumer application between a primary source, usually called the "normal" source, and a secondary source, usually called the "emergency" source. The normal source is most often a private or public supplier of electricity, generating electricity from a power station to many consumers. The emergency source is most often a generator located at the consumer's site, and is designed to supply electrical power for relatively short periods in the event of a failure at the normal source. Power is supplied from these sources through the switch to a load operated by a consumer.
Automatic transfer switches ar thus often found in hospitals and other consumer locations where a continuous supply of electrical power is required. A power failure or power reduction, without emergency backup, can have serious and possibly fatal consequences, and the reliability of the means for switching between power sources is of high importance. Moreover, because the time involved in transferring between the failed normal source and the emergency source must be short in such applications, it is necessary that the switching means for transferring between automatic and emergency is rapid. Automatic transfer switches employ a variety of power detecting means to achieve this automatic switching after a detection of some preset criterion (e.g., power loss or reduction). The portion of the bypass/isolation automatic transfer switch that engages in this switching operation is known as the automatic transfer switch.
Bypass/isolation automatic transfer switches having such characteristics are well known in the art. To maintain these characteristics, however, it is necessary that the switches are properly and regularly maintained. Also, like all electromechanical devices, automatic transfer switches are also occasionally in need of repair as a result of an electrical or mechanical failure. In both service environments, the person servicing the switch is required to open the automatic transfer switch and access or remove the actual switching unit connected to the normal and emergency power sources. High voltages and amperages typically pass through the automatic transfer switch, creating a hazardous environment for the service person and requiring the exercise of great care to maintain the safe operation of the automatic transfer switch.
When the bypass/isolation automatic transfer switch is serviced, power continues to be supplied from the primary or secondary source through a bypass/isolation switch. The bypass/isolation switch is typically, though not necessarily, located in the same physical housing as the automatic transfer switch.
The bypass/isolation switch is operated similarly to the automatic transfer switch, except that the transfer occurs as the result of manual switching, rather than automatic switching. A shift in operation from the automatic transfer switch to the bypass/isolation switch therefore requires that the bypass/isolation switch be connected to the same source as the automatic transfer switch, so that a continuous power supply is provided to the consumer. The automatic transfer switch may then be disconnected for service. After servicing, the automatic transfer switch is reconnected to the same source as the bypass/isolation switch, and the bypass/isolation switch is then disconnected so that power is supplied only through the automatic transfer switch.
This operation, however, presents several hazards against which caution must be taken. Particularly, it is necessary that the automatic transfer switch and the bypass/isolation switch be connected to the same source when both are supplying power to the load, since the primary and secondary sources can typically be expected to be out of phase with respect to one another. The hazards created by the connection of the out of phase sources through the automatic transfer and bypass/isolation switches include explosion, fire, electric shock and load equipment damage.
Previously, methods and mechanisms to avoid the improper connection of sources through the automatic transfer and bypass/isolation switches were generally mechanical, and more complex than that found in the present invention. These interlocks required the user to set numerous manual switches designating the locations of sources in both the bypass/isolation automatic transfer switch. This additional complexity resulted in not only an inefficient interlock system, but also greater risk of interlock failure and resultant hazards.